Cast nickel-base alloys and in particular the so-called nickel-base superalloys have been widely used in applications where resistance to high temperatures is required. Such applications are largely found in the hotter parts of gas turbine engines, in particular vanes and blades in aircraft engines. Superalloy castings have also been favoured for lower temperature (c. 600.degree. C.) applications for static structural parts such as casings, compressor and turbine exit guide vanes and bearing housings. For such applications, in addition to good creep resistance, weldability, fatigue resistance and low thermal expansion properties are required.
The compositions of such superalloys are chosen to meet specific engine requirements, and it is generally recognized that improvement in one property of a superalloy is usually at the expense of one or more other properties. For instance, it is difficult to make a nickel-base superalloy possessing good casting and welding properties whilst at the same time exhibiting high tensile strength and creep resistance.
Alloying elements in nickel-base superalloys have various roles, which may be summarised as follows.
Typically, nickel-base superalloys consist of the following phases:
1) Gamma matrix phase. This is typically high in nickel, chromium, cobalt, tungsten, and molybdenum. Rhenium and ruthenium may also be present in some applications. Nickel, cobalt, chromium, tungsten, molybdenum, and rhenium all affect the properties of the superalloy matrix.
2) Gamma prime precipitate strengthening phase. This is typically high in nickel, aluminum, titanium, niobium, tantalum, and vanadium. Some chromium and cobalt will be present. Hafnium will be present in the gamma prime phase in alloys that contain hafnium. The properties of the gamma prime phase are affected by the presence of these elements.
The gamma matrix is hardened by large, heavy, refractory elements (e.g. tungsten, molybdenum, rhenium) which distort the crystal structure--i.e. solid solution strengthening. The limits of addition of these elements is indicated by the onset of phase instability, where embrittling phases occur. This limit is predicted by a phase computation procedure which is known in the prior art whereby freedom from formation of embrittling phases is predicted if the composition has a low calculated value of the average electron vacancy number (Nv) of the matrix. Such refractory elements also slow down chemical diffusion which is beneficial for weldability and in controlling creep.
The gamma prime precipitate is hardened by the elemental content. The important feature of the precipitate is that it imparts strength to the matrix. The strength of the structure is a function of the amount of precipitate present, its size and shape distribution, and the stability of the structure in service. All of these factors are affected by the chemical balance.
Grain boundaries are strengthened by the presence of carbon, boron, hafnium and zirconium, and carbides such as those of chromium, tungsten, molybdenum, titanium, tantalum, niobium, vanadium, and hafnium.
It is desirable for good castability of a superalloy that it has a moderate freezing range of about 80.degree. C. to give low porosity. Low boron, zirconium, and carbon content gives hot tear and weld fissure resistance. A low carbide content during solidification gives low porosity.
Good weldability of a superalloy is indicated by a low aluminum/titanium ratio and low aluminum plus titanium total contents since this gives a low gamma prime volume fraction producing a weaker, more ductile alloy which is better able to accomodate the stresses produced during the weld thermal cycle. However, alloys of this nature are often weak and not suitable for higher performance turbine engine components.
Another approach is to employ precipitate strengthening elements (such as niobium) which have a low diffusivity in a low diffusivity matrix (i.e. containing refractory elements). This has been done in an alloy known in the prior art, IN718. This alloy, which is described in British Patent 2148323, has for a number of years been notably successful as a casting alloy used for many components in gas turbine engines. However, in order to operate designs at higher temperatures it is desirable to provide an alloy with higher temperature capability (IN718 is limited to about 650.degree. C.), higher strength and good weldability.
The benefit in strength over IN718 can be achieved by selecting a balanced chemistry (as described above) but it is necessary also to optimise the gamma prime volume fraction of the alloy such that weldability can be maintained. It is also necessary to optimise the gamma/gamma prime mismatch by controlling the refractory element content of the matrix/precipitate.
A low gamma/gamma prime mismatch leads to good precipitate stability and resistance to creep at high temperatures (greater than 800.degree. C.). However, for lower temperature operation a larger mismatch is preferred as strengthening is gained by the presence of large. coherency strains.
It is also known that a high chromium content limits the upper working temperature of the alloy, and this effect is usually counteracted by cobalt (as in the alloy IN939 which has a chromium content of 22% and a cobalt content of about 19%). It should be possible to gain a benefit in upper working temperature for an alloy by limiting the chromium content to about 16%, whilst still maintaining an adequate level of corrosion resistance.